The reference Fatigue Crack Growth (FCG) rate behaviour for austenitic stainless steel in a deaerated Pressurised Water Reactor (PWR) environment is provided in ASME Boiler and Pressure Vessel Code Section XI Code Case N-809. This FCG law is dependent on temperature, R ratio and environment; the environment effect is defined through a load rise time parameter. The basis for the law is contained within a large dataset of testing of simple waveforms carried out by various industry sources.
In a previous paper, PVP2016-63497, it was demonstrated that the use of load rise time as defined in Code Case N-809 did not provide a good characterisation of FCG rate for complex waveforms where the loading rate varies during the fatigue cycle. For waveforms representative of a thermal shock, the method in Code Case N-809 leads to overly conservative predictions of FCG rate. An improved method was postulated: the Weighted Stress Intensity Factor Rate (WKR) method. This method expresses the FCG law in terms of loading rate; a weighting is then applied to the effect of the loading rate based on its position within the fatigue cycle, according to the exponent on load. This method can be used to provide an effective rise time, which may be used in place of the rise time definition given in Code Case N-809. The WKR method was shown to give excellent correlation with test results for a number of complex waveforms.
This paper provides additional data and discussion related to the WKR method. This includes additional simple waveforms with two different loading rates (2-stage waveforms), several additional waveforms which are representative of PWR plant thermal shock loading and 2-stage waveforms at higher R ratios. Final confirmatory tests are also reported which were carried out at different temperatures and using different grades of stainless steel. These data address the gaps identified in PVP2016-63497. Having analysed the total evidence for the WKR method, the authors consider that there is a sufficiently strong basis for the method to be incorporated into calculations using Code Case N-809.
In order to assist in application of the WKR method, a simple worked example is also provided in this paper.